Presently, there is ever-increasing demand for semiconductor optoelectronic devices having spectral performances tuned for many critical applications. For example, FIG. 1 graphically shows the responsivity of a typical silicon-based photovoltaic detector. As shown, the silicon-based photovoltaic detector has an approximately linear spectral responsivity from about 200 nm to about 1200 nm. In contrast, FIG. 2 shows the spectral responsivity of the same photovoltaic detector tuned or otherwise configured to selectively detect a narrow wavelength band of incident light from about 720 nm to about 820 nm
In another example, FIG. 3 shows an example of the spectral output of a light emitting diode (LED). In contrast, FIG. 4 shows that the spectral output of the silicon-based LED shown in FIG. 3 may be tuned to output a more narrow spectral range. Presently, numerous applications demand precise spectral tuning of their optoelectronic components, including those used for high-precision biomedical fluorescence applications and/or other critical measurement-and-control applications.
Numerous methods have been attempted to produce spectrally tuned optoelectronic devices. For example, as shown in FIG. 5 one previous device incorporates an optically-coated glass component 5 mounted external of the housing 3 of the semiconductor device 1. Alternatively, FIG. 6 shows an embodiment of a device 7 having a detector device 11 positioned within a device housing 9. As shown, an optically coated glass filter device 13 is positioned within the housing 9 proximate to the detector device 11. Typical optically-coated glass devices consist of glass or glass-like optically transparent substrates (e.g. Schott Borofloat, BK-7, fused silica, etc) having at least one multilayer thin-film optical interference coating applied thereto. For example, often the multilayer optical interference coating comprises alternating layers of materials having a low index of refraction and a high index of refraction. While numerous methods exist for producing such optical thin-film coatings, relatively few coating processes achieve the film structure and density required for high-precision applications. For example, conventionally deposited optical thin films (e.g. thermal or electron-beam evaporated oxide-based materials such as SiO2, HfO2. Ta2O5, etc) have micro-morphologies which are columnar and porous, allowing the absorption and desorption of atmospheric moisture. As such, the effective index of refraction of the multi-layer thin film may change as a result of such moisture entrapment, which may result in spectral shifts and instabilities. For many critical instrument applications, this results in devastating losses of the optical precision. Alternative state-of-the-art optical coating processes therefore are geared towards densifying thin films as a means to prevent such moisture penetration. As such, the multilayer optical interference coating comprises alternating layers of low index and high index materials, wherein both the low and high index materials are high density materials. Such processes include reactive ion plating, ion-assisted electron-beam evaporation, ion-beam sputtering, magnetron sputtering, and plasma-enhanced CVD.
While current optical thin-film multilayer deposition technologies (e.g. ion plating, ion-beam sputtering, magnetron sputtering, ion-assisted electron beam deposition, CVD, etc.) satisfy the need for creating densified optical coatings, a number of shortcomings have identified. For example, these processes fail to provide the ideal configuration for use as directly deposited optical coatings upon semiconductor surfaces. More specifically, current deposition technologies produce densified optical coating films having undesirable excessive film stresses, which may deteriorate the performance of the device. In addition, these highly densified optical coating films are difficult, if not impossible, to further process (e.g. etch) once the coating is applied.
Therefore, in light of the foregoing, there is an ongoing need for a multilayer optical thin-film coating method capable of producing an optical coating on a semiconductor wafer device or material that is environmentally stable, but both minimizes harmful stresses, and allows for a simple, non-damaging and manufacturable post-deposition etching process.